Injection seeded F2 laser with line selection and discrimination

ABSTRACT

A narrow band F 2  laser system having two laser subsystems. The first laser subsystem is configured to provide a very narrow band pulsed beam at a first narrow wavelength range corresponding to a first natural emission line of the F 2  laser system. This beam is injected into the gain medium of the second laser subsystem in a first direction where the beam is amplified to produce a narrow band pulsed output beam. The seed laser subsystem also produces a second pulsed beam at a second wavelength range corresponding to a second natural emission line of the F 2  laser. This line is injected into the gain medium of the second laser subsystem in a second direction opposite said first direction. The second beam is amplified in the gain medium of the second laser subsystem depleting the gain medium of gain potential at the second wavelength range. (This amplified second beam is preferably wasted.) With the gain potential at the second undesired wavelength the range thus reduced the portion of light at the second wavelength range in the output beam is greatly reduced.

[0001] This Application is a Continuation-In-Part of Ser. No.09/855,310, filed May 14, 2001, Ser. No. 09/848,043 filed May 3, 2001,“Injection Seeded Laser with Precise Timing Control”, Ser. No.09/829,475 filed Apr. 9, 2001, “Injection Seeded F₂ Laser WithPre-Injection Filter”, Ser. No. 09/473,795 filed Dec. 28, 1999, “VeryNarrow Band Injection Seeded F₂ Lithography Laser”, Ser. No. 09/459,165filed Dec. 10, 1999, “Injection Seeded F₂ Lithography Laser”; 09/438,249filed Nov. 12, 1999, “F₂ Laser with Visible and IR Control”; Ser. No.09/421,701, filed Oct. 20, 1999, “Single Chamber Gas Discharge Laserwith Line Narrowed Seed Beam”, and Ser. No. 09/407,120 filed Sep. 27,1999, “Line Narrowed Laser with Etalon Output Coupler”, now issued asU.S. Pat. No. 6,240,110. This invention relates to lasers and inparticular to injection seeded lasers useful for integrated circuitlithography.

BACKGROUND OF THE INVENTION Prior Art Lithography Lasers

[0002] KrF excimer lasers are the state of the art light source forintegrated circuit lithography. Such lasers are described in U.S. Pat.No. 4,959,840, U.S. Pat. No. 5,991,324 and U.S. Pat. No. 6,128,323. Thelasers operate at wavelengths of about 248 nm. With the KrF laserintegrated circuits with dimensions as small as 180 nm can be produced.Finer dimensions can be provided with ArF lasers which operate at about193 nm or F₂ lasers which operate at about 157 nm. These lasers, the KrFlaser, the ArF laser and the F₂ lasers, are very similar, in fact thesame basic equipment used to make a KrF laser can be used to produce anArF laser or an F₂ laser merely by changing the gas concentration,increasing the discharge voltage and modifying the controls andinstrumentation to accommodate the slightly different wavelength.

[0003] A typical prior-art KrF excimer laser used in the production ofintegrated circuits is depicted in FIGS. 1, 1A and 1B. A cross sectionof the laser chamber of this prior art laser is shown in FIG. 1B. Asshown in FIG. 1A, pulse power system 2 powered by high voltage powersupply 3 provides electrical pulses to electrodes 6 located in adischarge chamber 8. Typical state-of-the art lithography lasers areoperated at a pulse rate of about 1000 to 2000 Hz with pulse energies ofabout 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1%fluorine, 1.3% krypton and the rest neon which functions as a buffergas) at about 3 atmospheres is circulated through the space between theelectrodes at velocities of about 1,000 to 2,000 cm per second. This isdone with tangential blower 10 located in the laser discharge chamber.The laser gases are cooled with a heat exchanger 11 also located in thechamber and a cold plate (not shown) mounted on the outside of thechamber. The natural bandwidth of the excimer lasers is narrowed by linenarrowing module 18 (sometimes referred to as a line narrowing packageor LNP). Commercial excimer laser systems are typically comprised ofseveral modules that may be replaced quickly without disturbing the restof the system. Principal modules include:

[0004] Laser Chamber Module,

[0005] High voltage power supply module,

[0006] High voltage compression head module,

[0007] Commutator module,

[0008] Output Coupler Module,

[0009] Line Narrowing Module,

[0010] Wavemeter Module,

[0011] Computer Control Module,

[0012] Gas Control Module,

[0013] Cooling Water Module

[0014] Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B issupported in this prior art embodiment by anode support bar 44 which isshown in cross section in FIG. 1B. Flow is counter-clockwise in thisview. One comer and one edge of anode support bar 44 serves as a guidevane to force air from blower 10 to flow between electrodes 6A and 6B.Other guide vanes in this prior art laser are shown at 46, 48 and 50.Perforated current return plate 52 helps ground anode 6B to the metalstructure of chamber 8. The plate is perforated with large holes (notshown in FIG. 3) located in the laser gas flow path so that the currentreturn plate does not substantially affect the gas flow. A peakingcapacitor bank comprised of an array of individual capacitors 19 ischarged prior to each pulse by pulse power system 2. During the voltagebuildup on the peaking capacitor, one or two preionizers 56 weaklyionize the lasing gas between electrodes 6A and 6B and as the charge oncapacitors 19 reaches about 16,000 volts, a discharge across theelectrode is generated producing the excimer laser pulse. Following eachpulse, the gas flow between the electrodes of about 1 to 2 cm permillisecond, created by blower 10, is sufficient to provide fresh lasergas between the electrodes in time for the next pulse occurring one halfto one millisecond later.

[0015] In a typical lithography excimer laser, a feedback control systemmeasures the output laser energy of each pulse, determines the degree ofdeviation from a desired pulse energy, and then sends a signal to acontroller to adjust the power supply voltage so that energy of thesubsequent pulse is close to the desired energy. These excimer lasersare typically required to operate continuously 24 hours per day, 7 daysper week for several months, with only short outages for scheduledmaintenance.

Injection Seeding

[0016] A well-known technique for reducing the band-width of gasdischarge laser systems (including excimer laser systems) involves theinjection of a narrow band “seed” beam into a gain medium. In one suchsystem, a laser called the “seed laser” or “master oscillator” isdesigned to provide a very narrow laser band beam and that laser beam isused as a seed beam in a second laser. If the second laser functions asa power amplifier, the system is typically referred to as a masteroscillator, power amplifier (MOPA) system. If the second laser itselfhas a resonance cavity, the system is usually referred to as aninjection seeded oscillator (ISO) and the seed laser is usually calledthe master oscillator and the downstream laser is usually called thepower oscillator.

Jitter Problems

[0017] In gas discharge lasers of the type referred to above, theduration of the electric discharge is very short duration, typicallyabout 20 to 50 ns (20 to 50 billions of a second). Furthermore, thepopulation inversion created by the discharge is very very rapidlydepleted so that the population inversion effectively exists only duringthe discharge. In these two laser systems, the population in thedownstream laser must be inverted when the beam from the upstream laserreaches the second laser. Therefore, the discharges of the two lasersmust be appropriately synchronized for proper operation of the lasersystem. This can be a problem because within typical pulse power systemsthere are several potential causes of variation in the timing of thedischarges. Two of the most important sources of timing variations arevoltage variations and temperature variations in saturable inductorsused in the pulse power circuits. It is known to monitor the pulse powercharging voltage and inductor temperatures and to utilize the data fromthe measurements and a delay circuit to normalize timing of thedischarge to desired values. One prior art example is described in U.S.Pat. No. 6,016,325 which is incorporated herein by reference. There inthe prior art timing errors can be reduced but they could not beeliminated. These errors that ultimately result are referred to as“jitter”.

F₂ Laser-Spectral Lines

[0018] A typical KrF laser has a natural bandwidth of about 300 pm(FWHM) centered at about 248 nm and for lithography use, it is typicallyline narrowed to about 0.6 pm. ArF lasers have a natural bandwidth ofabout 500 centered at about 193 nm and is typically line narrowed toabout 0.5 pm. These lasers can be relatively easily tuned over a largeportion of their natural bandwidth using the line narrowing module 18shown in FIG. 2. Also for the KrF and ArF lasers, the absolutewavelength of the output beam can be determined accurately by comparingits spectrum to atomic reference lines during laser operation. F₂ laserstypically produce laser beams with most of its energy in two narrowlines centered at about 157.63 nm and 157.52 nm.

[0019] Often, the less intense of these two lines (i.e., the 157.52 nmline) is suppressed and the laser is forced to operate at the 157.63 nmline. The natural bandwidth of the 157.63 nm line is pressure dependantand varies from about 0.6 to 1.2 pm. An F₂ laser with a bandwidth inthis range can be used with lithography devices utilizing a catadiophiclens design utilizing both refractive and reflective optical elements,but for an all-refractive lens design the laser beam should have abandwidth to produce desired results. It is also known that thecenterline wavelength of the output beam will vary somewhat depending oncondition in the discharge region.

[0020] Lasers for lithography equipment are very complicated andexpensive. Further reduction in bandwidth could greatly simplify thelens design for lithography equipment and/or lead to improved quality ofintegrated circuits produced by the equipment. Thus, a need exists forlithography lasers (including KrF, ArF and F₂ lasers) with substantiallyreduced bandwidth.

Bandwidth Control

[0021] The wavelength of KrF and ArF lasers is relatively easilycontrolled over ranges of a few hundred picometers corresponding totheir natural bandwidths. The F₂ laser or the other hand has in partbeen considered untunable since the a large portion of its output isconcentrated in two narrow lines. Several techniques have been preparedfor selecting one of the lines and eliminating energy in the other line.

Amplified Spontaneous Emissions

[0022] In many line selected laser systems, such as the KrF and ArFsystems, a buildup in a narrow frequency band suppresses buildings inother spectral regions. However, weak interaction between the variouslaser frequencies may reduce the effect of this mechanism. F₂ lasersystems provide very large gains allowing substantial intensity build-upor merely a single pass through the gain medium. Initial light levelsmake originals as so-called spontaneous emissions at any potential laserline. When these spontaneous emissions are amplified, the light isreferred to as amplified spontaneous emissions (ASE) and it may become apart of the output laser beam reducing the quality of the beam. In thecase of the F₂ laser many efforts have been made to suppress the 157.52nm line while maintaining efficient production of the desired 157.63 nmline. For use as a lithography light source, there is a desire to reducethe intensity of the 157.52 nm line to less than O1.% of the 157.63 nmline. What is needed is a better F2 laser system in which the 157.52 nmline is suppressed to insignificance.

SUMMARY OF THE INVENTION

[0023] The present invention provides a narrow band F₂ laser systemhaving two laser subsystems. The first laser subsystem is configured toprovide a very narrow band pulsed beam at a first narrow wavelengthrange corresponding to a first natural emission line of the F₂ lasersystem. This beam is injected into the gain medium of the second lasersubsystem in a first direction where the beam is amplified to produce anarrow band pulsed output beam. The seed laser subsystem also produces asecond pulsed beam at a second wavelength range corresponding to asecond natural emission line of the F₂ laser. This line is injected intothe gain medium of the second laser subsystem in a second directionopposite said first direction. The second beam is amplified in the gainmedium of the second laser subsystem depleting the gain medium of gainpotential at the second wavelength range. (This amplified second beam ispreferably wasted.) With the gain potential at the second undesiredwavelength the range thus reduced the portion of light at the secondwavelength range in the output beam is greatly reduced. In a preferredembodiment a pulse power supply is provided which is speciallyconfigured to precisely time the discharges in the two laser subsystemso that the discharges are properly synchronized, and laser gascomprises F₂ at a partial pressure less than about 1% with a buffer gascomprised of helium or neon or a combination of helium and neon. Controlof center wavelength of the output beam may be provided by adjusting oneor more of the following parameters in the first laser: the total lasergas pressure, the relative concentration of helium or neon, F₂ partialpressure, laser gas temperature, discharge voltage and pulse energy.

[0024] For precise jitter control in preferred embodiments include apulse power system with a pulse transformer unit having two sets oftransformer cores. A single upstream pulse compression circuit provideshigh voltage pulses in parallel to the primary windings of all of thecores in both sets. Separate secondary conductors (one passing throughone set of cores and the other passing through the other set of cores)provide very high voltage pulses respectively to separate downstreamcircuits supplying discharge pulses to the electrodes in each of twoseparate laser chambers. In preferred embodiments line narrowing isaccomplished within the resonant cavity of the seed laser and/or theoutput of the seed laser could be line narrowed using a pre-gain filter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a drawing of a prior art commercial excimer lithographylaser.

[0026]FIG. 1A is a block diagram showing some of the principal elementsof a prior art commercial excimer lasers used for integrated circuitlithography.

[0027]FIG. 1B is a drawing of the laser chamber of the FIG. 1 laser.

[0028]FIG. 2 is a block diagram showing features of the presentinvention.

[0029]FIG. 3 is an electrical circuit drawing showing features of apreferred embodiment of the present invention.

[0030]FIG. 3A is a drawing of a pulse transformer.

[0031]FIG. 3B is a drawing showing features of FIG. 3A.

[0032]FIG. 3C is a modification of the FIG. 3 circuit providing anadjustable delay.

[0033]FIG. 3C1 is a B-H curve.

[0034]FIG. 3D shows an alternate filter adjustment technique.

[0035]FIGS. 4 and 5 are black diagrams of F₂ laser systems.

[0036]FIGS. 6 and 6A show features of a first grating monochromator.

[0037]FIG. 7 shows features of a second grating monochromator.

[0038]FIG. 8 shows features of an etalon filter.

[0039]FIGS. 9 and 10 are diagrams of power gain stages.

[0040]FIGS. 11, 11A, 11B and 11C show features of a pulse power system.

[0041]FIG. 12 shows a pulse energy detector in a feedback controlsystem.

[0042]FIGS. 13 through 17 show preferred embodiments of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Applicants' Experiments

[0043] In order to understand and characterize the effect of variationsin various F₂ laser operating parameters on the centerline wavelengthand bandwidth of F₂ laser systems, Applicants have conducted carefulexperiments and have determined precisely absolute values of the twomajor lines of the F₂ laser beam spectrum. Applicants have alsodetermined that the centerline wavelength of the F₂ laser can becontrolled to some extent by controlling laser operating parameter,especially laser gas pressure. Bandwidth can also be controlled to anextent by controlling these parameters.

[0044] A summary of all this test data is set forth below in Table I.TABLE I Summary of F₂ Laser Center Wavelength Data Absolute wavelength(D2 reference spectrum, 200 kPa He buffer): Strong line: 157.63090 nmWeak line: 167.52418 nm Pressure shift coefficients: He: 1.84 fm/kPa Ne:0.81 fm/kPa Zero pressure 157.63053 nm wavelength: Central wavelength atarbitrary helium/neon buffer: λ = 157.63053 + 1.84 × 10⁻⁶P_(He) + 0.81 ×10⁻⁶P_(Ne)<nm> Parametric sensitivities: Expected effect HV: 1 fm/V 20fm F₂ −20 fm/kPaF₂ 1 fm Temperature: −0.6 to −1.3fm/° C. 4-8 fm Energy:2.2 fm/mJ 5 fm

[0045] Applicants' summary conclusion from these tests are:

[0046] 1) The determination of a “zero pressure” wavelength and pressureshift coefficients allows for a universal specification of the centralwavelength.

[0047] 2) The central wavelength can be inferred to sufficient accuracythat a wavemeter is not needed.

[0048] 3) Variability in the central wavelength of the F₂ laser, due tonormal changes in laser operating parameters, appears limited to ±0.02pm

[0049] 4)

Injection Seeded F₂ Laser System Seed Laser

[0050]FIGS. 2 through 11D describe various techniques for designing andoperating injection seeded F₂ laser systems. An F₂ injection seed lightsource may be a conventional F₂ laser, using either a plane-paralleloptical resonator, or an unstable resonator configuration. The pulsepower supply for both laser subsystems is preferably provided utilizingone of the techniques described above. This assures that the timing ofthe discharge of each laser subsystem is adequately synchronized. In apreferred F₂ embodiment the seed laser beam is filtered downstream ofthe seed laser. The seed laser preferably will generate enough energysuch that, after filtering, 10-100 μJ of narrow-band energy is availablefor seeding the F₂ power gain stage. An unstable resonator will producea lower divergence, more spatially coherent beam than a stableresonator, which may be of some advantage in coupling energy through theinjection spectral filter. For instance, if the filter is a simplemonochromator, a lower divergence beam will be more easily focused downto the input slit of the monochromator. Another design option is tooperate the first F₂ light source laser at relatively low pressure(≈100-200 kPa). This produces a substantially reduced spectral width:0.3-0.6 pm. A lower spectral width means a greater fraction of theenergy entering the post gain filter will make it through the filter.The raw output energy from the first F₂ light source will be much lower,but this may not be a practical disadvantage because the maximum energythat the injection filter can handle is similarly limited.

[0051] A conventional F₂ laser for use as the first F₂ light source in apreferred embodiment a standard KrF lithography laser system modifiedfor operation as a F₂ laser. These KrF lithography lasers are well knownand there are more than 1,000 of these units operating today inintegrated circuit fabrication plants as light sources for integratedcircuit lithography machines. These lasers produce laser pulses at ratesin the range of 1000 to 2000 pulses per second and are available fromsuppliers such as Cymer, Inc. with offices in San Diego, Calif. Theselithography lasers are described in detail in many patents such as U.S.Pat. No. 5,991,324 and U.S. Pat. No. 6,128,323 both of which areincorporated herein by reference. The major modifications needed foroperation as an F₂ laser are to change the gas mixture to about 0.1percent fluorine and the remainder helium (although a neon or acombination of helium and neon could be used) and preferably the upperrange of the discharge voltage is increased from about 26,000 volts toabout 36,000 volts. A basic prototype F₂ laser system used for both thefirst F₂ light source and the power gain stage is described below in thesection entitled “F₂ Laser System Designs”. That section describes thesignificant improvements to the prior art KrF laser system to produce anF₂ laser.

Pre-Power Gain Filter

[0052] One technique for producing a narrow band laser beam is to filterthe output of the seed laser beam prior to injecting it into theamplifier stage. A conventional grating-monochromator pre-power gainfilter is described by reference to FIG. 6. This filter, when used witha free-running F₂ laser as the master oscillator, preferably must sliceout a 0.1 pm bandwidth portion of the free-running spectrum from themaster oscillator, and be capable of producing the 10-100 μJ ofnarrow-band energy required by the following amplifier stage. Light fromthe master oscillator is first focussed down onto an input slit 50.Light passing though the input slit is collimated preferably by a curvedmirror 54, which may be a simple spherical mirror, or an off-axisparaboloid, and the collimated light is directed to a grating 54. Thegrating is a high dispersion type (e.g. an echelle grating) chosen todisperse light in the 157 nm wavelength range. The grating is in theLithrow configuration. Light at a selected very narrow range which isreflected back along the beam path 54 is re-imaged on an exit slit 57with the aid of beam splitter 56. The various geometric and opticalparameters of the arrangement (i.e. slit widths, grating dispersion,curved mirror focal length) determine the bandwidth of the light leavingthe exit slit. One design problem that must be overcome is the high peakintensities that are reached at the input and exits slits when oneattempts to couple the desired amount of energy through themonochromator. One method to handle these high intensities is to userefractive slits, i.e. knife edge wedges that refract the unwanted lightinto another direction, without absorbing the energy. Such a slitarrangement is shown in FIG. 6A.

[0053] In addition to its function as a filter, the arrangement of FIG.6 includes an additional beamsplitter 58 and a linear detector array 60placed at an exit image plane. This addition solves an importantremaining problem: how to maintain the (tunable) injection filter at thedesired wavelength. If the grating angle is in error by more than a few10's of micro-radians, the seed beam will miss the exit slit, and therewill be no narrow-band energy to lock the following power amplifierstage. If the angle is very slightly incorrect the amplified spectrumwill not be at the desired wavelength. Fortunately, the monochromatorcan, in essence, monitor itself. The image formed by the grating andcurved mirror is the dispersed spectrum of the light entering themonochromator. The second beam splitter produces two identical images(spectra), one at the exit slit, and one at the linear detector array.The linear detector array senses the relatively broad spectrum from thefree-running master oscillator and converts it into a video signalrepresentative of the spectral intensity at each point on the array.Since the free-running wavelength is stable and well-known, it serves asa calibration standard for the monochromator. A controller 66 reads outthe linear detector array, and adjusts the grating angle so as to placethe image of the spectrum centered at a desired point on the array, nearthe center for example. In this way the monochromator is self-stabilizedto the free-running spectrum of the master oscillator. Since the exitslit and linear array are basically duplicate image planes, the exitslit position corresponds to a particular position, and hencewavelength, on the linear array. Therefor, with the free-runningspectrum from the master oscillator as a spectral reference, thewavelength of the light leaving the exit slit can be preciselydetermined.

[0054] One method for calibrating this arrangement is to place abeamsplitter 62 in the path of the beam exiting the monochromator and tomonitor the beam energy with energy detector 64. Such a detector isdesirable in any event, since the energy of the injection seed needs tobe monitored.

Calibration

[0055] A calibration sequence would proceed as follows: (1) With thegrating at a starting angle, the laser is fired and the output energyfrom the exit slit is monitored, along with the spectral image fallingon the linear array. The peak of the spectrum is determined in terms ofthe position, in pixels, on the array. (2) The grating angle isincremented and the measurements are repeated. (3) After the gratingangle is scanned though a range, the resulting data is examined. Theposition of the spectrum on the array (in pixels) where the outputenergy maximizes corresponds to the equivalent position of the slit.

[0056] Once this calibrated position is determined, the known dispersionof the monochromator can be used to retune the grating to otherwavelengths. For instance, suppose the monochromator dispersion is 0.1pm/pixel, and further suppose the calibrated position of the exit slitis pixel 300. If the desired wavelength of the output is 157.6299 nm(157,629.9 pm), the center of the free-running spectrum, then thegrating angle is adjusted so that the center of the image falls at pixel300. If the desired wavelength is ±0.2 pm away from the center (157.6301nm), then the grating would be moved so that the center of the imagewould fall at pixel 302. A further refinement is to include the pressureof the master oscillator in the calibration and subsequent use of themonochromator, since the center of the free-running spectrum is pressuredependent. This pressure dependence must be included in the calibration,especially if the pressure of the master oscillator is allowed to varysignificantly. We have determined that the center wavelength of thefree-running laser has a pressure shift coefficient in the range ofabout 1 to 2 fm/kPa, when helium is used as a buffer. For any givenpressure, a good estimate of the wavelength is therefor157.6309+0.00000192* P nm, where P=pressure in kPa. Other pressure shiftcoefficients can be used if other buffer gasses are used (neon forinstance, or mixtures of helium and neon).

Modified Grating Monochromator

[0057] An alternate method for producing the narrow-band light is with amodified grating monochromator as shown in FIG. 7. This filter filtersthe output beam of a master oscillator which produces a well-collimated,nearly diffraction limited coherent beam, and in this case we eliminatethe entrance slit of the monochromator.

[0058] A beam expander 70 is used to reduce the divergence from themaster oscillator and to physically match the size of the beam from themaster oscillator to grating 54. The dispersed light from the grating isfocussed via a curved mirror (or lens) to an exit slit 72 where thedesired wavelength is selected. The operation of the linear detectorarray 60 and controller 66 is the same as previously described. Theadvantage of this arrangement is that it eliminates the need for anentrance slit and the associated problems with high peak intensities.This arrangement has a disadvantage in that the pointing stability ofthe master oscillator is now a factor in the position of the image onthe array, and hence the wavelength selection process. For slowlyvarying changes in the input angle from the master oscillator, thecontroller can retune the grating and keep the wavelength constant.

Etalon Filter

[0059] An etalon 78 can also be used as a bandpass filter as shown inFIG. 8. As with the monochromator filters, it is desirable that theetalon be self-referenced to the free-running spectrum of the masteroscillator, which is used as an atomic standard. The beam from themaster oscillator is first expanded with beam expander 70, both to lowerits divergence and to reduce the power density on the etalon. Afterexpansion, the beam passes through a special “partial diffuser” 74, anoptical element which transmits most of the light unaltered, butscatters a small faction into a range of angles. Examples of this arediffractive optics with low diffraction strength, or very lightly andfinely ground optical flats. The light then passes through the etalon atnear normal incidence. The etalon's bandpass characteristics aredetermined by its free spectral range (FSR) and finesse. For instance,an etalon with an FSR of 2 pm and a finesse of 20 will have a bandpassof 0.1 pm FWHM. The etalon will then transmit a 0.1 pm slice of thespectrum from the free-running master oscillator. As with themonochromator, it may be advantageous to operate the master oscillatorat low pressure, and hence reduced bandwidth, in order to limit thepower loading on the etalon. In addition, a narrower starting spectrumwill reduce the amount of energy in the adjacent transmission orders ofthe etalon (±1 FSR from the central wavelength). After passing throughthe etalon, a lens 76 (or curved mirror) focuses the light to a point,where an aperture is placed. A portion of the beam is split off by beamsplitter 80 and monitored by a photodiode array 82 which provides centerwavelength and bandwidth signals to controller 66 which uses thisinformation to control the etalon 78. The purpose of the aperture is toblock all light except the on-axis, non-diffused component of the beam.This light, which is narrow-band, is then sent on to the power amplifierstage. A beam splitter and optical detector follows the aperture, tomonitor the energy leaving the injection filter.

[0060] In general, the central wavelength of the etalon's bandpass willnot align with the center of the spectrum of the free-running masteroscillator. The etalon needs to be tuned. Four methods are possible,depending on whether the etalon is a solid plate type or an air-spacedtype. For solid etalons, the etalon can be tilted with respect to theincoming beam, or the temperature of the etalon can be varied, whicheffectively changes the optical thickness of the plate. For air-spacedetalons, the angle can be varied, the plate spacing can be varied (byPZT actuators, for instance), or the index of refraction of the gasbetween the plates can be varied by changing the density of the gas.

[0061] In a preferred embodiment, the etalon is an air-spaced type whichis enclosed in a pressure-tight housing 84 as indicated in FIG. 8. Apressure controller is used to vary the pressure of the gas within thehousing (at constant temperature), thereby pressure tuning the etalon.Since the amount of tuning required is very small (about ±0.2 to ±0.5pm), the pressure change that is needed is also very small, about ±3 to±8 torr with nitrogen as the gas. This can be achieved either bychanging the volume of the (sealed) housing, or by actively introducingor withdrawing gas from a suitable supply. As the etalon is pressuretuned, the output intensity will alternately increase and extinguish asthe bandpass wavelength sweeps through the spectrum from thefree-running master oscillator.

[0062] Control over the tuning of the etalon is attained by includingadditional optical elements that turn the etalon into its own etalonspectrometer. An additional beamsplitter is located prior to the exitaperture, and forms a second focal plane of the lens. In this focalplane (as in the first), the intensity distribution consists of anintense spot at the center of focus plus a much weaker conventionaletalon ring pattern. The intense spot is formed by the non-diffusedportion of the beam passing through the etalon, whereas the etalon ringpattern is formed by the diffused portion of the beam. The intensecentral spot is not used here and is blocked with a beam stop 81. Alinear detector array 82 is then placed in the focal plane to read outthe etalon ring pattern. This arrangement is very similar to the designof current wavemeters used in lithographic lasers. For a given opticalarrangement, there is a direct relationship between the diameters of theetalon rings and the central wavelength of the etalon bandpass.

[0063] Calibration of the arrangement is done in a manner similar to theprocedure previously described for the monochromator filter. Acalibration sequence would proceed as follows: (1) With the etalonpressure- tuned to a starting wavelength, the laser is fired and theoutput energy from the exit aperture is measured, along with the etalonring pattern falling on the linear array. The diameter of the innermost,fully formed ring is determined. (2) The pressure controller incrementsthe pressure in the etalon, and the measurements are repeated. (3) Afterthe etalon is pressure-tuned through one free-spectral range, theresulting data is examined. The diameter of the innermost ring where theoutput energy maximizes corresponds to the condition where the bandpassof the etalon is exactly tuned to the peak of the free-running spectrumfrom the master oscillator which is pressure dependent.

[0064] Once this calibrated diameter is determined, the wavelength ofthe etalon bandpass filter can be stabilized by varying the pressure inthe etalon so as to maintain this diameter. As a further refinement, theetalon ring pattern can be converted directly into a “wavelength” byemploying the same non-linear etalon equations used in the lithographicwavemeters. This allows the bandpass function to be detuned from thepeak of the master oscillator spectrum by a known amount. A furtherrefinement is to include the pressure of the master oscillator in thecalibration and subsequent use of the etalon, as has been previouslydescribed.

Tuning

[0065] Normally the narrow spectral band (of about 0.1 pm or less)chosen for operation of the laser system will be the narrow band meetingdesired bandwidth specifications with the maximum output pulse energy.However, a limited amount of wavelength tuning is possible with thepregain filters described above. Applicants expect that a tuning rangeof at least about 1.2 pm can fairly easily be achieved. Additionaltuning is feasible with some significant compromise of output pulseenergy. The tuning range is a function of laser gas pressure asindicated above. Therefore a longer overall range can be achieved byadjusting the pressure in the laser.

Power Gain Stage

[0066] Two preferred power gain stages can be described by reference toFIGS. 9 and 10.

Power Oscillator

[0067] The power gain stage can be configured as a power oscillator asshown in FIG. 9. Many different resonators can be used for the POdesign, depending on the desired output. In a preferred embodiment, theresonator is an off-axis hybrid unstable resonator, formed by twosplit-coated mirrors, all as shown in FIG. 9.

[0068] The injected seed beam 90 is aligned to a central axis along thetop of an unstable resonator 92, and first travels through a 50% partialreflector 93A. The rear resonator optic 94 is a zero power meniscustype, which does not disturb the collimation of the injected beam. Theinjected beam fills the Fresnel core of the resonator, establishingcontrol over the intercavity field (the Fresnel core in this case is thevolume formed between 50% R mirror 93A and 100% R mirror 94A). Afterpropagating with amplification to the front optic, the beam is reflectedfrom the 100% reflective, convex surface. The beam expands and isamplified as it travels to the rear optic, where a portion reflects offof the 100% reflective, concave surface 93B and a portion reflects offsurface 93A. This re-collimates the beam, which is further amplified bya third pass through the gain. The lower portion 94B of the outputcoupler is anti-reflective coated, allowing the beam to exit withminimal loss. As with the rear optic, the front optic is a zero-powermeniscus type, to preserve the collimation of the output beam. This typeof a resonator forms a power oscillator because the 50% and 100%reflective surfaces provide feedback into the Fresnel core of theresonator. The advantage of this type of resonator is (1) there is nocentral obscuration or hole in the beam, and (2) it requires very littleseed energy to lock the power oscillator to the seed.

Power Amplifier

[0069] A power gain stage in the form of a power amplifier is shown inFIG. 10. In this case the resonator is similar to the one shown in FIG.9 except the feedback has been eliminated by changing the 50% reflectivesurface 93A to an anti-reflective surface. This configuration producesan off-axis, multipass power amplifier.

F₂ Laser System Designs

[0070] Several prototype F₂ laser systems have been built and tested byApplicants and their fellow workers to serve as gain media for both asthe first F₂ light source, and as the power gain stage.

[0071] These systems are largely based on current production KrF and ArFlasers incorporating several important improvements over prior artexcimer laser systems, utilizing a high efficiency chamber andsolid-state pulsed power excitation. The discharge is corona pre-ionizedto minimize gas contamination. The entire optical beam path is nitrogenpurged to avoid light absorption by oxygen and to avoid damage tooptical components. All resonator optics were external to the angledchamber window equipped laser chamber. The gas mixture was 0.1% fluorinein 4 atmospheres of helium and the electrode gap was reduced to 10 mm.

[0072] In these prototype units, for both the first F₂ light source andthe power gain stage, a modified pulse power system is used and acircuit diagram for the system is shown in FIG. 11 and in FIGS. 2 and 3.The major difference between the pulse power system for theseembodiments and corresponding systems for prior art KrF lasers is thepulse transformer 56 which for the F₂ laser provides higher outputvoltage and, as described above, the portion of the circuit upstream ofthe transformer is a single circuit and the portion downstream of thetransformer is a divided circuit. In this pulse transformer the singlefour-section stainless steel rod (described in U.S. Pat. No. 6,128,323referred to above) which functions as a secondary winding is replaced bya transformer secondary conductor consisting of an inner cylindrical rodand two coaxial tubes all connected in series and insulated from eachother as shown in FIGS. 11A, 11B and 11C. The secondary conductorconsists of two coaxial assemblies (the cross-section of which aredepicted in FIGS. 11B and 11C) connected with bus bar as shown at 302and HV cable as shown at 304. FIG. 11D shows the same cross-sections as11B and 11C and also the layers 306 of Metglas™ and mylar film which iswrapped around the cylinder portion 308 of the spools forming theprimary winding. Also identified in FIG. 11D are the central wire 310and hollow cylindrical conductors 312 and 314 forming the secondaryportion of the pulse transformer. The Metglas™ and mylar layers are notshown in FIGS. 11A, 11B and 11C. A current pulse having a voltage peakof about 1,000 volts (as indicated at 316) will produce a pulse at thesecondary HV terminal of about 36,000 volts as indicated at 318 in FIG.11A. Thus, each of the two pulse transformer sections feeding the twolasers, is comprised of 12 induction units (instead of the 23 shown inFIG. 3HB). However, the three secondary conductors passing through the12 induction units produce voltage amplification of 36.

[0073] Coupling between the primary cylinders and the three coaxialsecondary conductors is provided by wrappings of Metglas™ and mylar filmas described above with reference to FIG. 8E. In this embodiment anextra stage of compression (with one additional capacitor bank C_(p−1))is provided. The capacitor banks in this embodiment have the followingvalues: C₀ = about 12.1 μF C₁ = about 12.4 μF C_(p-2) = about 8.82 nFC_(p-1) = about 8.4 nF C_(p) = about 10 nF

[0074] The modified pulse power system in this prototype embodimentproduces an output rise time of about 80 ns into the peaking capacitorbank. The step-up ratio of the pulse transformer is 36× (as compared tothe 24× in the embodiment described in detail above). This permits thelaser to operate at substantially higher voltages with correspondinglylower F₂ concentrations as compared to the unmodified pulse transforms.Applicant has determined that the higher voltage operation improvesdischarge stability, and permits higher repetition rates. As explainedabove, in this embodiment two separate transformers are provided each ofwhich are supplied by a primary current from a common source as shown inFIG. 3B but each of the transformers are configured a shown in FIGS.11A, B, C and D to provide a 36× step up instead of the 24× as shown inFIG. 3B.

Post Output Filter

[0075] As indicated above, the output of power gain stage of preferredembodiments of the present invention will have ultraviolet bandwidths ofabout 0.1 pm or less with a line center within the nominal F₂ 157.63 nmline which covers a spectral range of about ±0.5 pm around the nominalwavelength. As indicated in the following section, a small amount oflight energy in other spectral ranges are produced in F₂ lasersespecially red and infrared light when helium is used. If this red lightis a problem, it can be easily eliminated with well know optical filtersdesigned to transmit 157 nm UV light and absorb or reflect away (notback into the laser) the red light. Also, a post output filter of one ofthe types described above could be added to further line narrow theoutput beam in the UV range. However, when used as a post output filter,the components of the filter need to be designed to handle a much higherenergy beam.

Pulse Timing

[0076] Preferred embodiments of the present invention utilize a pulsepower system configured to control the timing of the discharge of thetwo laser systems to produce desired output pulse laser beams. Theseembodiments make use of a fractional turn pulse transformer designsimilar to fractional turn pulse transformer described in U.S. Pat. No.5,142,166. In these embodiments the portion of the pulse power circuitsfor the two laser systems are separate downstream of the pulsetransformer system and portions of the pulse power circuits upstream ofthe pulse transformer system is common for both laser systems.

[0077]FIG. 3 shows an electrical outline of the principal elements of apreferred pulse power system. The portion of the system upstream of thepulse transformer system is very similar to the circuit shown in FIG. 11which is described in detail U.S. in U.S. Pat. No. 6,151,346 (herebyincorporated by reference). A single bank of capacitors define acharging C₀ capacitor bank 42. Electrical pulses are generated by theclosing of switch S₁ 46 which consists of two IGBT switches mounted inparallel L_(o) inductor 48 holds off current flow through S₁ so it canclose without any deterioration to charge up C₁ capacitor bank 52. L₁saturable inductor holds off substantial current flow through pulsetransformer system 56 until capacitor L₁ saturates at which time theprimary turns of fractional turn pulse transformer system 56 is pulsedwith a short about 0.5 microsecond 1000 volt pulse. In this embodiment,transformer system 56 is comprised of two separate transformer units 56Aand 56B which in this case are virtually identical.

[0078] Each of the pulse transformer unit 56 is similar to the pulsetransformer described in U.S. Pat. No. 6,151,346. The pulse transformerunits of the present embodiment has only a single turn in the secondarywinding and 23 induction units. The transformer as configured as an autotransformer as shown in FIG. 3B to provide a 1:24 step-up ratio. Each ofthe 23 induction units comprise an aluminum spool 56A having two flanges(each with a flat edge with threaded bolt holes) which are bolted topositive and negative terminals on printed circuit board 56B as shownalong the bottom edge of FIG. 3A. (The negative terminals are the highvoltage terminals of the twenty three primary windings.) Insulators 56Cseparates the positive terminal of each spool from the negative terminalof the adjacent spool. Between the flanges of the spool is a hollowcylinder 1{fraction (1/16)} inches long with a 0.875 OD with a wallthickness of about {fraction (1/32)} inch. The spool is wrapped with oneinch wide, 0.7 mil thick Metglas™ 2605 S3A and a 0.1 mil thick mylarfilm until the OD of the insulated Metglas™ wrapping is 2.24 inches. Aprospective view of a single wrapped spool forming one primary windingis shown in FIG. 5 of U.S. Pat. No. 6,151,346.

[0079] The secondary of each transformer is a single OD stainless steelrod mounted within a tight fitting insulating tube of Teflon® (PTFE).The transformer units are in four sections as shown in FIG. 3A. The lowvoltage end of stainless steel secondary shown as 56D in FIG. 3A is tiedto the primary HV lead on printed circuit board 56B at 56E, the highvoltage terminal is shown at 56F. As a result, the transformer assumesan auto-transformer configuration and the step-up ratio becomes 1:24instead of 1:23. Thus, an approximately −1400 volt pulse between the +and − terminals of the induction units will produce an approximately−35,000 volt pulse at terminal 56F on the secondary side. A 1000 voltprimary pulse produces a pulse on the secondary sides of bothtransformers of about 24,000 V. This single turn secondary windingdesign provides very low leakage inductance permitting extremely fastoutput rise time.

[0080] The general configuration of the pulse transformer system isshown in FIG. 3B. As indicated in this figure, the primary high voltagepulse of about 1000V produced by the upstream portion of the pulse powersystem arrives at each pulse transformer at exactly the same time and asthe corresponding output pulse of each of the transformers willtherefore be substantially identical in shape and time. Applicantsestimate that the jitter at the output of the two transformer will beless than one nanosecond.

[0081] As indicated in FIG. 3 in this embodiment the portion of pulsepower circuits downstream of the pulse transformers are separate butsubstantially equal so that the jitter at electrodes 83 and 84, A and B,is estimated to be less than 3 ns. Therefore, the gain medium in bothlasers is produced at the same time with a variation of less than about3 ns. The duration of each of the pulses is about 20 to 50 ns so thatthe laser pulse produced in the first laser is properly amplified in thesecond laser. Preferably, the circuit is provided with a bias circuit tobias all saturable inductors so that they are reverse conducting priorto each pulse. The bias circuit is designed so that during a shortperiod immediately after the pulse the saturable inductors remainforward conducting so that pulse energy reflected from the electrodescan be recovered as explained in detail in U.S. Pat. No. 5,729,562.

[0082] In preferred embodiments of this invention, the output coupler ofthe first laser is located about one foot downstream of the input windowof the second laser. Therefore, for this reason or for other reasons, itmay be desirable to delay the discharge of the second laser as comparedto the first laser. Since the electrical pulse travels through a goodconductor at a rate of about ins/20 cm, this can easily be accomplishedby making a conductor carrying the pulse of the second laser longer (forexample, by 20 to 40 cm) than a corresponding conductor for the firstlaser.

Adjustable Delays

[0083] Another approach to control the timing of the discharge in onelaser relative to the other is to insert a saturable inductor in thecircuit shown in FIG. 3 in one of the branches downstream of transformer56 such as at location 63 shown in FIG. 3C. This saturable inductor isfitted with an adjustable forward bias. The forward bias which isapplied is chosen so that the time to complete the forward saturation ofthe inductor is approximately equal to the desired delay time. The delaytime is a function of the number of turns in the saturable inductor, thecross-section of its magnetic core and magnetic flux swing ΔB of theinductor. Since the required delay is very small the number of turns canbe one and the core can be small (such as 2 inch diameter) and the fluxswing ΔB can also be small as indicated in FIG. 3C2. By adjusting thebias the relative delay can be adjusted. The delay control could beincorporated into a feedback loop design to control jitter. Since thedelay expected to be required for an oscillator/amplifier configurationis small (on the order of ns or 10's of ns), the delay reactor can bemade small. In addition, the core material can be selected to minimizethe losses introduced in the circuit, again since the volt-secondrequirement is likely to be much smaller than that required in the powerpulse compression circuit. Another technique for providing for anadjustable delay is shown in FIG. 3D. In this case, a conductor 101carrying the pulse to one of the lasers is arranged in a single loopcoil 102 and a rod 103 having high permeability arranged to be movableinto and out of the coil. The rod can be positioned with a fast drivesuch as a stepper motor or a piezoelectric driver.

[0084] Still another way of providing an adjustable delay is shown inFIG. 3E. In this technique, pulsed current source 86 is used to providea secondary pulse current for inductor L₄. This current source 86 istriggered by a trigger circuit 86, which also closes the switches.Current pulse I_(s) starts at about the same time as switch S closes andlasts a time t_(d) after the main compressed current pulse propagates tocapacitors C₄ and C₄′. t_(d) can be about 10 us. The secondary pulsedcurrent serves to provide a saturation for inductor L₄. By changing thiscurrent, a saturation point on BH curve of inductor L₄ can be changed,causing the delay to change as well. The corresponding inductor in theamplified circuit, L₄ is baised with a non-adjustable bias currentI_(bias). Other embodiments of this technique are also possible, such asproviding two pulsed current sources for inductors L₄ and L₄′.

Other Pulse Transformer Configurations

[0085] Many variations of the pulse transformers configuration shown inFIG. 3B are possible. The preferred output voltage for the two lasersystems may not be the same. If different voltages are needed this canbe easily accomplished by providing a smaller or larger number ofinduction units for one of the transformers relative to the other one.Also, switches could be included in one of the transformers to cut outsome of the induction units to reduce the discharge voltage output ofthat transformer relative to the other one. Taps could be providedbetween any of the four transformer sections of either transformer totake off the secondary voltage at reduced levels.

CONTROLLING THE CENTERLINE WAVELENGTH

[0086] Applicants have done extensive testing to explore techniques forcenterline wavelength control of F₂ lasers systems as described above.Applicants have determined that in a master oscillator, power amplifierconfiguration where the seed beam is in the range of 20 μJ to 50 μJ orgreater the center wavelength and the band width of the seed beamdetermines almost exactly the center wavelength and band width of theoutput beam. Applicants have also measured precisely the effects oncenter wavelength and band width of laser gas pressure, buffer gas mix,F₂ partial pressure, laser gas temperature, discharge voltage and pulseenergy. Applicants then use these results to control the centerwavelength by varying one or more of the above parameters. The degree ofparameter charge necessary to make the centerline wavelength isindicated by the pressure shift coefficients shown in Table I above.

[0087] Applicants believe that the total laser gas pressure will be themore easily used parameter to control centerline wavelength, but theother parameters could be used or a combination of more than oneparameter could be used.

Line Selection With Discrimination

[0088]FIGS. 13 through 17 described preferred embodiments of the presentinvention in which the 157.63 nm line is selected and the 157.52 nm lineis actively discriminated. FIG. 13 depicts a first embodiment of thepresent invention. A seed laser 100 is configured as a low-pressuremaster oscillator for reduced bandwidth, a second laser 102 isconfigured as an efficient, high-pressure-operated power amplifier. Theseed laser 100 employs a resonant cavity comprised of laser chamber 101,partial reflective mirrors 104 and 106, permitting extraction of laserradiation on both ends of the resonant cavity. Prisms 108 and 110 atboth ends angular disperse the two dominant laser lines of the F₂ laser.The adjustment of these prisms is such as to inject the desired stronglaser line at one end of power amplifier 102 via mirror 114 and toinject the undesired laser line at the other end via partial mirror 116.Beam stops 118 prevents the weak line radiation of the seed laser fromentering into the entrance of power amplifier 102. Similar, beam stop120 prevents the strong line radiation of the seed laser from enteringinto the power amplifier from its output side. With this design, lowbandwidth light from the seed laser on the strong laser line will beefficiently amplified in power amplifier 102. In order to reduce theundesired ASE output on the weak line, active gain extraction isprovided by means of the corresponding output of the seed laser from theother side. The weak line radiation is amplified within the poweramplifier, substantially reducing the potential gain in the spectralregion of the weak line, which in turn reduces ASE in this spectralregion in the output beam 124. The amplified radiation of the undesiredlight finally passes through prism 110, where dispersion allowsseparation from the optical axis of the seed laser. The energy on thiswavelength is dissipated in beam stop 122. Obviously, the dispersion ofboth prisms has to be large enough to ensure sufficient discriminationwith the corresponding beam stop. This requires the angular separationto be larger than the intrinsic divergence of the seed laser beam onboth ends. Single prisms of CaF₂ material, transparent at these VUVwavelengths, can achieve approximately 0.5 mrad angular dispersionbetween both laser lines, while maintaining reasonable transmissionefficiencies. Since the gain in power amplifier 102 is very large, seedlaser 100 can be operated with divergence limiting apertures and thuswith reduced efficiency. Another method is a master oscillator designwith single- or dual axis unstable resonator design, which is known toachieve divergences below 0.5 mrad easily. Multiple prisms could be usedinstead of single prisms on both ends of the master oscillator toincrease dispersion if preferable for a particular implementation.

[0089]FIG. 14 shows a different, diffraction grating based design. Seedlaser 126 is again configured as a master oscillator with a resonantcavity comprising high reflective mirror 128 and partial reflector 130.The seed laser output, containing both wavelengths, is directed towardsgrating 132, which is used to disperse both wavelengths. The undesiredwavelength is cleanly separated with a slit 134 and injected into thefront of power amplifier 136 gain generator via turning mirrors 138 and140, the latter preferably is an non-coated CaF₂ substrate at Brewster'sangle, so that optical losses from the power amplifier throughput areminimized. Since the seed laser radiation is not or only weaklypolarized, coupling via Brewster's angle is possible. The amplifiedlight of the undesired wavelength is propagating backward via mirror 142to grating 132, where dispersion directs it into beam stop 144. Thedesired wavelength selected with slit 146 and directed into the entranceof the power amplifier via turning mirror 142. The advantage of thisparticular design is the increased dispersion of the grating,significantly easing the separation of the two wavelengths.

[0090]FIG. 15 shows an embodiment employing polarization dependentoptical retardation/polarization rotation. In this embodiment,polarizers 150 and 152, and partial mirrors 154 and 156 form seed laser158. One of the polarizers might be omitted under situations of moderategain, i.e. at lower operating pressures, in generator laser chamber 101.A phase retarder 160 is used to rotate the undesired wavelengthpolarization by 90 degree while keeping the polarization of the desiredwavelength unchanged. A subsequent polarizer 162 rejects transmissionsof the undesired wavelength as it has perpendicular polarization. Suchretardation-based systems, made of bi-refringent materials, such as MgF₂and polarizers, are called Lyot filters and have been used in wavelengthseparation applications for many years, i.e. in astronomicalinvestigations, employing relatively large beam divergences. Theremaining light of the desired wavelength is passed into the entrance ofthe power amplifier chamber 103 via turning mirror 164 and polarizingbeam splitter 166. On the opposite end of the seed laser a similararrangement separates the weak line from the output. Phase retarder 168and polarizer 170 select the weak line to be passed into the front endof chamber 103 of power amplifier 176 via mirror 172 and partialmirror/beam splitter 174. The difference in wavelength selection isachieved by rotation of polarizer 170 by 90 degrees. For this reason,the amplified light of the undesired wavelength propagating backwards inthe power amplifier has perpendicular polarization with respect to thedesired wavelength propagating forward. This feature is used to separateand dissipate backward propagating energy with polarizing beam splitter166 and beam stop 178 respectively. Advantageous with this design is theinherent polarization of the power amplifier output, which is a resultof the polarized input radiation.

[0091]FIG. 16 shows a simplified scheme, which uses only a one-sidedphase retarder/Lyot filter between the seed laser and the poweramplifier 182. The seed laser consists of laser chamber 101, mirror 184polarizer 186 and partial mirror 188. The polarized light is objected tophase retarder 190, which separates the two wavelengths in terms oftheir polarization. While the polarization of the desired laser line iskept unchanged, the undesired is rotated by 90 degree. Both laser linesare directed into the entrance of laser chamber 103 of power oscillator182 via turning mirrors 192 and 194 which are not required if thelongitudinal extension allows lining-up the two gain generator stages. Apolarizing beam splitter 196 at the output discriminates the weak linedue its 90 degree rotated polarization and directs it into beam stop198. The desired wavelength passes polarizing beam splitter 196practically without loss.

[0092] To further reduce the bandwidth and to spectrally purify thesystems output radiation, etalons might replace one or both of theresonator mirrors of the seed laser section. The general idea is verysimilar to the previously described examples 1, 2 and 3. Since the gainof the power amplifier is not completely homogeneously broadened, partof the spectrum emitted can be suppressed by injection of the accordingspectra into the front of the power amplifier gain generator.

[0093]FIG. 17 shows a scheme, which is very similar to the FIG. 1 schemeexcept it employs an etalon 200 at one end of the seed laser 100. Theetalon transmission minimum is tuned to the desired wavelength range,such as to reduce the propagation of this spectral content into thefront of the power amplifier gain generator 103. At the same time, thetransmittance of the undesired spectral content can be maximized byappropriate choice of the free spectral range (etalon spacing). Thisundesired spectral content is directed into the front of the poweramplifier. The particular tuning of the etalon also maximizes thereflection of the desired wavelength within the seed laser resonatortowards the power amplifier entrance, while minimizing the amount of theundesired spectral content propagating into this direction.

[0094] Further improvement from the scheme depicted in FIG. 17 can beachieved with scheme where the second seed laser mirror 104 is replacedwith an etalon 202 as well. However, this etalon will be tunedcomplementary to the first etalon by appropriate choice of its freespectral range. This arrangement has improved selectivity/discriminationof the propagation of desired and undesired spectral contents within thesystem, as now two selective elements are involved.

[0095] Generally, a variety of similar optical schemes are possible, allof them providing the undesired spectral content into the front of thepower amplifier thus depleting the optical gain and associated ASE inforward/output direction in the corresponding spectral range. Thedesired spectrum is generally provided at the power amplifier entrancefor efficient amplification.

[0096] Several advantages are achieved with these designs:

[0097] 1. Narrowband output

[0098] 2. High spectral purity

[0099] 3. High efficiency due to low, i.e. no post-filtering required tosuppress weak line.

[0100] Ideally the arrangement is made symmetrical, that is, thepropagation time of light (equivalent with distance) from the seed laserinto the power amplifier is made the same by appropriate optical layout.This way the gain medium of the power amplifier will interacts with bothlaser lines simultaneously, though from different directions, therebymaximizing wavelength discrimination. However, even quite largedeviations from symmetry are allowed as the temporal gain profile is atleast several ns wide, which corresponds to distance differences in therange of 1 m.

[0101] In fact, a slight advance of the light injection on the outputside of the power amplifier might be used to reduce the red/IR outputradiation in the same manner. The gain on these laser transitions isalso very high (super-radiant emission) which in turn requires verylittle amounts, even from stray light levels of input intensity tosaturate the gain. Temporal gain and emission on these lines occursabout 10-20 ns earlier than the VUV gain/laser pulse, which suggests toinject the lines earlier from the front.

[0102] Although the present invention has been described in terms ofspecific embodiments, the reader should understand that the scope of theinvention is to be delimited by the appended claims and their legalequivalents.

We claim:
 1. An injection seeded F₂ laser system comprising: A) a seedlaser subsystem comprising a gain medium produced by electric dischargesthrough a laser gas comprising fluorine and a buffer gas, said seedlaser being configured to produce: 1) a first narrow band laser beam ata desired first narrow wavelength range corresponding to all or aportion of a first natural emission line of said seed laser subsystem,and 2) a second laser beam at at least one undesired wavelength rangecorresponding to a second natural emission line of said seed lasersubsystem; B) a second laser subsystem comprising a second gain mediumproduced by electric discharges through a laser gas comprising fluorineand a buffer gas; C) a first optical train configured to direct saidfirst desired narrow band laser beam into said second gain medium foramplification into a narrow band laser system output beam, and D) asecond optical train configured to direct said second laser beam intosaid second gain medium so as: 1) to deplete said second gain medium ofgain potential in said undesired wavelength range, and 2) to reduce, insaid laser system output beam, light intensity at the undesiredwavelength range.
 2. A laser system as in claim 1 wherein said seedlaser subsystem comprises a centerline wavelength control means foradjusting one or more of a group of laser parameters consisting of:laser gas pressure, buffer gas mix, F₂ partial pressure laser gastemperature, discharge voltage and pulse energy.
 3. A laser system as inclaim 2 wherein said centerline control means comprises a means tocontrol laser gas pressure.
 4. A laser system as in claim 1 and furthercomprising a pulse power system comprising: A) a pulse transformersystem comprising: 1) a first pulse transformer comprising: a) a firstplurality of transformer cores defining a number of cores N, each corehaving a primary winding, b) at least one first secondary conductor,passing through all of said first plurality of cores, 2) a second pulsetransformer comprising: a) a second plurality of cores defining a numberof cores M, each core having a primary winding; b) at least one secondsecondary conductor, passing through all of said second plurality ofcores, B) a high voltage pulse power source for producing high voltageelectric pulses of relatively long duration, C) an upstream electricalpulse compression circuit for compressing said high voltage electricalpulses to produce compressed high voltage pulses of relatively shortduration, said upstream circuit being configured to apply saidcompressed high voltage pulses in parallel: 1) to said primary windingof each of said first plurality of transformer cores and 2) to saidprimary winding of each of said second plurality of transformer cores,to produce very high voltage first pulses at an output on said firstsecondary conductor and to produce very high voltage second pulses at anoutput on said second secondary conductor, D) a first downstreamelectrical circuit for applying said first very high voltage pulses tosaid first set of electrodes to create discharges in said firstdischarge region, and E) a second downstream electrical circuit forapplying said second very high voltage pulses said second set ofelectrodes to pulse to create discharges in said second dischargeregion, wherein said first laser subsystem output beam is amplified insaid second discharge region to produce an amplified laser beam at anoutput of said second discharge laser subsystem.
 5. A laser system as inclaim 4 wherein N is equal to M.
 6. A laser system as in claim 4 whereinN is not equal to M.
 7. A laser system as in claim 4 wherein N and M areeach approximately equal to
 23. 8. A laser system as in claim 4 whereinsaid first secondary conductor is a single conductor and said secondsecondary conductor is a single conductor.
 9. A laser system as in claim4 wherein said at least one first secondary conductor is a plurality ofcoaxial conductors and said at least one secondary conductor is aplurality of coaxial conductors.
 10. A laser system as in claim 4 andfurther comprising a pulse delay means for delaying one of said veryhigh voltage first pulses and very high voltage second pulses withrespect to the other.
 11. A laser system as in claim 10 wherein saidpulse delay means comprises an elongation of a conduction path.
 12. Alaser system as in claim 10 wherein said delay means comprises anadjustable bias on a saturable indicator.
 13. A laser as in claim 4 andfurther comprising a saturable inductor filtered with an adjustableforward bias.
 14. A laser as in claim 4 and further comprising means fordetecting jitter and a jitter control feedback loop.
 15. A laser systemas in claim 1, wherein: A) said second laser system is configured as apower amplifier; B) said first optical train comprises a prism beamseparator configured to separate light at said desired wavelength fromlight at said undesired wavelength range, so as to create said firstnarrow band laser beam at said desired first narrow wavelength range,said first narrow band laser beam being injected into said second gainmedium in a first direction; and C) said second optical train comprisesa prism beam separator configured to separate light at said undesiredwavelength range from light at said desired wavelength range in order tocreate second laser beam, said second laser beam being injected intosaid second gain medium in a second direction opposite said firstdirection.
 16. A laser system as in claim 1 wherein said seed lasersubsystem comprises a grating configured to produce said first narrowband beam and said second narrow band beam.
 17. A laser system as inclaim 1 and further comprising at least one Lyot filter for producingsaid first narrow band beam and said second narrow band beam.
 18. Alaser system as in claim 17 wherein said first narrow band beam definesa first polarization and said second narrow band beam defines a secondpolarization rotated about 90 degrees from said first polarization andboth beam are injected into said second gain medium in a singledirection and separated after amplification in said gain medium by apolarizing beam splitter.
 19. A laser system as in claim 1 wherein saidsecond optical train also comprises an etalon arranged to reduceintensity of light at the desired wavelength range in the second laserbeam.